(1) Field of the Invention
The present invention generally relates to exhaust gas recirculation systems, and more particularly to an exhaust gas recirculation system for internal combustion engine which recirculates a small amount of exhaust gas into air/fuel mixture for slowing combustion speed in each combustion chamber of engine cylinders and lowering the peak combustion chamber temperature so that the amount of nitrogen oxide (NOx) included in the exhaust gas is reduced.
(2) Description of the Related Art
Conventionally, an exhaust gas recirculation system is used with an internal combustion engine, especially a gasoline engine for automobile vehicle employing gasoline fuel, for reducing the amount of nitrogen oxide (NOx) included in exhaust gas from the internal combustion engine. In general, the exhaust gas from the internal combustion engine contains several kinds of harmful gas including carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx). By using the exhaust gas recirculation system, it is possible to inject a small amount of the exhaust gas into the intake air/fuel mixture so that inert gas included in the exhaust gas absorbs a part of thermal energy generated owing to combustion in each of engine cylinders, thereby lowering the peak combustion chamber temperature in each engine cylinder and reducing the amount of nitrogen oxide (NOx) included in the exhaust gas. Such a function being achieved by the exhaust gas recirculation system is hereinafter called an exhaust gas recirculation.
However, there is a problem in that an excessive amount of exhaust gas being recirculated to the air/fuel mixture in the intake system results in an unfavorable decrease in the output power of the internal combustion chamber and an unstable combustion in each combustion chamber of engine cylinders, which will cause poor driveability and undesired increase in the amount of hydrocarbon (HC) included in the resultant exhaust gas. To eliminate such a problem, it is necessary to suitably control the quantity of exhaust gas recirculated to air/fuel mixture in an intake system in accordance with a driving condition of the internal combustion engine. The quantity of the recirculated exhaust gas is hereinafter called an EGR quantity. For this reason, there is a conventional exhaust gas recirculation system in which the exhaust gas recirculation is stopped to promote an engine idling for an increased engine temperature, when fuel is poorly evaporated at an engine temperature that is lower than a prescribed reference value of engine temperature, thereby stabilizing the driving condition of the engine. Such a conventional exhaust gas recirculation system is disclosed, for example, in Japanese Laid-Open Patent Application No. 51-129527 and in Japanese Laid-Open Utility Model Application No. 49-58921.
In general, fuel which is commercially available, for use in an internal combustion engine, especially in an automobile gasoline engine, may be classified by the distillation characteristic into several different types, for example, a light type fuel, a heavy type fuel and an ordinary fuel. A fuel of the type suitable for practical use is selected for individual internal combustion engines. In the following description, therefore, consider a criterion for the classification of fuel into such different types depending on whether more than 50% of the fuel evaporates at 100 deg. C. In such a case, the light type fuel is in conformity with this criterion and the heavy type fuel is not in conformity with this criterion. The light type fuel generally contains many parts having a low boiling point below 100 deg. C, while the heavy type fuel contains many components having a high boiling point above 100 deg C. Especially in a case of an automobile gasoline engine, several different types of fuel having different distillation characteristics may be used for the automobile gasoline engine. This is, the light type fuel sometimes may be used and the heavy type fuel in the other times may be used. Accordingly, the amount of the fuel in a liquid state sticking to an intake manifold wall and flowing without being evaporated into vapor in the case of the heavy type fuel is greater than that in the case of the light type fuel. The heavy type fuel does not easily evaporate when compared with the case of the light type fuel, and there is a greater part of the heavy type fuel sticking to an inside wall of an engine intake port in a liquid state, and the amount of such fuel liquid is greater than that in the case of the light type fuel.
The amount of fuel vapor which actually enters a combustion chamber of the internal combustion engine is equal to the amount of fuel vapor injected by the fuel injection valve to the combustion chamber from which the amount of fuel sticking to the inside wall of an intake port in a liquid state is subtracted. The amount of the fuel sticking to the inside wall of the intake manifold in a liquid state is highly unstable. As described above, the heavy type fuel includes such a kind of fuel parts in a greater percentage. In a case of the heavy type fuel, therefore, the composition of the fuel is varied greatly for each cycle of engine operation, and the fuel does not enter the combustion chamber of the internal combustion engine in a constant amount for the cycles of engine operation, thereby causing an undesired variation of air/fuel ratio within the combustion chamber and an instability of engine operation which are more appreciable in a case where the light type fuel is used. Therefore, when the heavy type fuel is used, it is necessary to set the reference value of engine temperature, at which the exhaust gas recirculation is stopped, to a higher level. In the meantime, when the light type fuel is used, the light type fuel easily evaporates and the engine operates in a relatively stable condition. Therefore, when the light type fuel is used, it is desired to set the reference value of engine temperature at which the exhaust gas recirculation is stopped, to a lower level, so that the amount of nitrogen oxide (NOx) included in the exhaust gas is reduced by the exhaust gas recirculation.
However, in a conventional exhaust gas recirculation (EGR) system, the reference value of engine temperature at which the exhaust gas recirculation is stopped is preset to a fixed value, regardless of what type of fuel is being used with the internal combustion engine. Therefore, in a case where the reference value of engine temperature is preset to a level that is suitable for use of the light type fuel, a poor driveability is often caused when the heavy type fuel is used with the internal combustion engine. On the other hand, in a case where the reference value of engine temperature is preset to a different fixed level that is suitable for using the heavy type fuel, reduction of the amount of nitrogen oxide (NOx) in the exhaust gas by the exhaust gas recirculation is unfavorably performed when the light type fuel is used with the internal combustion engine.